Direct viewing storage tube arrangements producing &#34;black and white&#34; pictures



Jan. 12, 1965 PRODUCING "BLACK AND WHITE" PICTURES Filed Jan. 15, 1962 2 Sheets-Sheet 2 j-Pl m; 9 WWW F762. m

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DIRECT VIEWING STORAGE TUBE ARRANGEMENTS PRODUCING "BLACK AND WHITE" PICTURES 2 Sheets-Sheet 1 Filed Jan. 15, 1962 United States Patent Ofiice 3,165,665 Patented Jan. 12, T965 DCT VIEWING STORAGE TUBE MENTS PRODUCING BLACK AND TE PICTURES Jervois Campbell Firmin, Little Baddow, Essex, and Ian Robertson Sinclair and Norman John Davies, Chelmsford, Essex, England, assignors to English Eiectric Valve Company Limited, London, England, a British company Filed Jan. 15, 1962, Ser. No. 166,3ii3 Claims priority, application Great Britain, Jan. 13, 1961, 1,522/61 7 Claims. (Cl. 315-12) This invention relates to storage tube circuit arrangements and more specifically to arrangements including storage tubes of the kind comprising an electric charge storage structure having a backing grid or mesh with an insulator on one side thereof, a so-called writing gun adapted to scan said structure with a signal-modulated electron beam to produce on said insulator stored electric charges controlled by said signals, a so-called flood gun adapted to cover the working area of said structure with a collimated substantially uniform density flood beam of electrons, and a fluorescent viewing screen positioned to receive electrons of the flood beam which pass through said structure. As is Well known, with such a tube, the stored charge pattern on the storage structure determines whether or not flood beam electrons will pass through it at any particular point to reach the viewing screen on which, accordingly, appears a visible picture determined by the signals. For the sake of brevity tubes of this kind will hereinafter be referred to as direct viewing storage tubes.

The commonly employed known circuit arrangemnts including direct viewing storage tubes result in the production of what may be termed half-tone pictures, the stored charges being representative of and approximately proportional to the modulating signals so that there are gradations of light and shade between fully bright and fully dark in the visible pictures produced. There are, however, substantial advantagesmainly in the direction of longer storage and clearer and brighter produced pictures--to be obtained by substituting for half-tone pictures, what may be termed, by analogy, black and white pictures, i.e. pictures all parts of which are either fully bright, with a predeterimned approximately constant brightness, or fully dark. This result may be achieved by a circuit arrangement which will cause the tube to operate in a bistable manner so that signals above a predetermined strength applied to modulate the writing beam will result in stored charges which will cause flood beam electrons to pass through the storage grid or mesh at full strength at the points where such charges are stored while signals below said predetermined strength will cause the appropriate points on said structure substantially to cut off the flood beam electrons. The present invention seeks to provide improved, simple and reliable circuit arrangements which will produce such bi-stable operation.

Acording to this invention a storage tube circuit arrangement comprises a direct viewing storage tube of the determined potential such as to permit the passage of flood beam electrons therethrough. Preferably the flood gun cathode potential is held constant and said recurrent potentials applied to the backing grid or mesh. Advantageously, these potentials are applied as successively different valued pulses, the amplitudes of which are so chosen in relationto the secondary emission ratio-voltage characteristic curve of the storage mesh that, in the absence of stored charges, the storage mesh is stabilized at a predetermined potential sufficient to prevent the passage of flood beam electrons and, in the presence of stored charges, the storage mesh is stabilized at a potential tha permits the passage of flood beam electrons. Preferably there aretwo recurrent potentials and preferably, also, the recurrent frequencies tials are equal and constant. 1

Preferably said two different potentials occur for equal time periods and are so chosen that in the absence of stored charges the larger of said potentials alone produces a secondary emission ratio, in relation to flood beam electrons, which is approximately as much above unity as the corresponding secondary emission ratio produced by the smaller of said two potentials alone is below unity. Preferably the aforesaid potentials are applied in the form of two time-spaced approximately rectangular pulses. This, although preferred, is not a necessity and other waveforms inwhich the required two potentials successively occur may be used. For example there may be a single pulse having two different successive potential values such as would be obtained by reducing to zero the time spacing between the aforesaid two approximately rectangular pulses. i

There is a wide choice open as regards the recurrence frequency of the. different potentials, the time interval between the occurrences of the successivelyoccurring potentials, and also the durations of the successively occurring potentials. In general the repetition frequency should be high enough not to result in visible flicker of the produced pictures and the duration of and time interval between the successively occurring potential values should be so chosen as to result in a duty ratio (ratio of duration of the said potential values to the repetition period) such as will result in pictures of the required brightness and good light-dark contrast. The higher the duty ratio the higher the background brightness will be I but selection of duty ratio is not critical and, in practice,

with a given tube and operating conditions, a satisfactory duty ratio is easily selected by trial and error.

The invention is illustrated in and further explained in connection withtheaccompanying drawings, inwhich FIGURE 1 is a schematic simplified view of one embodiwriting gun audaflood gun on one side of said mesh,

kind referred to, means for modulating the writing electron beam of said tube by means of input signals and means for applying between the backing grid or mesh, which has an insulator on one side thereof, and the flood gun cathode of said tube different successively recurrent potentials so chosen in relation to the secondary emission rangement comprises a cathode FC and electrodes FGl,

through and, in the presence of stored charges produced by said signals, said insulator is stabilized at a second preand a fluorescent screen F8 on the other 'side of said mesh, the screen being usually, as shown, on theinside of I the end wall of the envelope and having on the side nearer the mesh a metal backing plate to which, in use, a posi tive potential is applied The writing gun arrangement comprises a cathode WC, control electrodes or so-called grids WGl, WGZ and WG3 (the last being for focussing) and mutually perpendicular deflecting pairs of plates D for causing the writing beam spot to scan over the stor age mesh in a predetermined manner; The flood gun ar- FGZ and F63, F83 being constituted by wall deposit and FG2 being in part constituted by wall deposit. A second ary electron collector mesh CM arranged to collect secondary electrons from the storage mesh SM is positioned on the gun side of the storage mesh adjacent thereto.

of both said potenr Signals to be displayed are applied from a signal source SS to the writing gun control electrode WGl to modulate the scanning Writing beam in intensity and a recurrent waveform consisting of two approximately rectangular pulses of different amplitudes and equal durations are applied from a source PS together with a D.C. potential, whose value is best found by trial and error, to the backing electrode (not separately shown) of the storage mesh SM.

The upper part of FIGURE 2 is a desired characteristic curve connecting secondary emission ratio R (ratio of emitted secondary electrons to incident primary electrons) as ordinates, with voltage V of the insulating surface of the storage mesh referred to the flood gun cathode potential as abscissae, of a direct viewing storage tube such as is shown in FIGURE 1. The horizontal line X represents unity secondary emission ratio. In the lower part of FIG- URE 2 is represented, in conventional manner, the effects of pulses applied from the source PS. In this lower part of the figure, the vertical axis is time, T and against it, horizontally, are plotted values of voltage V at the insulating surface of the storage mesh SM referred to the potential of the flood gun cathode, assuming the flood gun to be in operation. Assume for the moment that no charges have been written on the mesh. Then if a succession of small voltage pulses of suitable amplitude like that of the pulse P1 is alone applied to the storage mesh backing electrode, the insulating surface thereof will receive electrons from the flood beam during the occurrence of the pulses and will set itself to a negative voltage, approximately equal to the amplitude of the pulses, of, say 4 volts, such voltage being more negative than that corresponding to black (i.e. fully dark) on the viewing screen. Under these conditions no flood gun electrons will pass through the mesh to the viewing screen in the intervals between the pulses. This is the initial tube-setting operation.

Now suppose a recurrent waveform composed of two time spaced approximately rectangular pulses of different amplitude as shown by the pulses P1 and P2 are supplied to the backing grid or mesh. Still assuming no signals to be written on the storage electrode, the amplitude of pulse P2 is such as to produce a secondary emission ratio R just above unity (i.e. the amplitude is such that the insulator potential just exceeds the so-called first crossover potential at which the secondary emission characteristic curve of FIGURE 2 crosses the line X), with the result that during the occurrence of pulse P2 the landing of flood beam electrons on the storage insulator causes its potential to increase positively. The succeeding pulse P1 will now drive the storage insulator positive by a small amount such as to produce a secondary emission ratio R which is less below unity than the comparable value of R during pulse P2 was above it. As a result the potential of the storage insulator drifts negatively during the occurrence of pulse P1 but by an amount which is less than the positive drift during the occurrence of pulse P2. Owing to the fact that the secondary emission characteristic of the insulator surface has a slope, at a potential of V, which is of opposite sense to, and greater in Value than, that at the first crossover potential, a point is reached at which the positive drift during pulse P2 is compensated by the negative drift during pulse P1 (i.e. pulse P2 produces a secondary emission ratio R which is as much above unity as that due to pulse P1 is below it), and the insulator surface adopts a substantially stabilized potential, between pulses, corresponding to black on the Viewing screen. This situation is illustrated in FIGURE 2 by that part of the drawing above the line W. (As shown Pl produces a ratio of about 0.75 and P2 a ratio of about 1.25.)

Now suppose the writing gun, whose cathode potential is below that of the flood gun cathode and such as to pro vide a secondary emission ratio of greater than unity, is modulated to Write a pattern of positive charges on the storage mesh. The part of FIGURE 2 below the line W indicates the effect which one of these stored charges will have-namely equally to increase the potential attained by those areas of the storage mesh insulating surface at which writing has taken place during the occurrence of the two pulses. As will be seen the value of the ratio R at the top of the smaller pulse will no longer be as much below unity as the value of said ratio at the top of the larger pulse is above it owing to the flattening off of the secondary emission characteristic between zero and the first crossover potential. Consequently there is a tendency for the aforesaid areas of the storage insulator to become more positive. This tendency is, however, over come due to the fact that the written areas of the insulator surface, between pulses, have a potential which is below the first crossover potential and are consequently cathode potential stabilized whereby they rapidly reach zero potential as is illustrated in the drawing by the dashed line. When zero potential is reached the amount of positive charge acquired by the storage insulator during pulse P2 still exceeds that lost during pulse P1 but the insulator is held at zero potential due to cathode stabilization. Flood beam electrons will of course pass through the storage mesh under these conditions and consequently the written areas will be represented as white on the viewing screen.

If the writing beam is not of sufiicient intensity to drive the Written areas of the storage insulator positive those areas will nevertheless adopt a potential which is. less negative than that of the unwritten areas.

if the increase in potential of the written areas is above a predetermined small value, the peak of pulse P2 will correspond to a value of secondary emission ratio R which is in excess of unity by an amount greater than that by which the value of R corresponding to the peak of pulse P1 is below unity. Consequently, the written areas will increase positively in potential until they are stabilised at zero potential.

Thus where charges are not written on the mesh, its insulating surface will assume a potential cutting off the electrons of the flood beam from the screen and whereever charges have been written the flood beam electrons will pass through the mesh to the screen with an intensity which is, for practical purposes, independent of the modulating signal strength at the writing gun. Accordingly, a black and white (as distinct from a half-tone) visible picture is produced.

It will be appreciated that the pulses P1 and P2 need not be of equal duration, in which case the amplitudes of the pulses need not be such as to correspond, when the insulator surface of the storage electrode carries no written charge, to values of secondary emission ratio R which are equally and oppositely disposed about unity but will have values such that, in these conditions, the amount of positive charge acquired by the insulator during the larger pulse equals that lost during the shorter pulse. The amplitudes of the pulses must also be such that when the insulator is at flood-gun cathode potential the amount of positive charge acquired during the larger pulse exceeds that lost during the smaller pulse.

Furthermore, the different potentials applied to the backing grid or mesh may be in the form of a single pulse such as would be obtained if the time-spacing between the aforesaid two pulses, whether of equal duration or not, were reduced to zero.

Stored signal patterns written on the mesh can obviously be expunged when desired by omitting the larger amplitude pulses in the recurrent double pulse waveform. If desired, means may be provided for adjusting the amplitude of one or both of the pulses included in the waveform.

We claim:

1. A storage tube circuit arrangement including a direct viewing storage tube comprising an electric charge storage structure having a backing grid or mesh with an insulator on one side thereof, a writing gun adapted to scan said structure with a signal-modulated electron beam to produce on said insulator stored electric charges controlled by said signals, a flood gun adapted to cover the working area of said structure with a collimated substantially uniform density flood beam of electrons, and a fluorescent viewing screen positioned to receive electrons of the flood beam which pass through said structure, means for modulating the writing electron beam of said tube by means of input signals, and means for applying between the backing grid or mesh and the cathode of said flood gun successively diflerent recurrent potential values so chosen in relation to the secondary emission ratio-voltage characteristic curve of said insulator that, in the absence of stored charges produced by said signals, said insulator is stabilized at a predetermined potential suflicient to prevent the passage of flood beam electrons therethrough and, in the presence of stored charges produced by said signals, said insulator is stabilized at a second predetermined potential to permit the passage of flood beam electrons therethrough.

2. An arrangement as claimed in claim 1 wherein the flood gun cathode potential is held constant and said successively different recurrent potential values are applied to the backing grid or mesh.

3. An arrangement as claimed in claim 1 wherein there are two recurrent potential values.

4. An arrangement as claimed in claim 3 wherein the recurrence frequencies of both said recurrent potential values are equal and constant.

5. An arrangement as claimed in claim 3 wherein said two different recurrent potential values occur for equal time periods and are so chosen that in the absence of stored charges'the larger of said potential values alone produces a secondary emission ratio, in relation to flood beam electrons, which is approximately as much above unity as the corresponding secondary emission ratio pro- References Cited by the Examiner UNITED STATES PATENTS 2,843,799 7/58 Hook et a1 315l2 DAVID G. REDINBAUGH, Primary Examiner. RALPH G. NILSON, Examiner. 

1. A STORAGE TUBE CIRCUIT ARRANGEMENT INCLUDING A DIRECT VIEWING STORAGE TUBE COMPRISING AN ELECTRIC CHARGE STORAGE STRUCTURE HAVING A BACKING GRID OR MESH WITH AN INSULATOR ON ONE SIDE THEREOF, A WRITING GUN ADAPTED TO SCAN SAID STRUCTURE WITH A SIGNAL-MODULATED ELECTRON BEAM TO PRODUCE ON SAID INSULATOR STORED ELECTRIC CHARGES CONTROLLED BY SAID SIGNALS, A FLOOD GUN ADAPTED TO COVER THE WORKING AREA OF SAID STRUCTURE WITH A COLLIMATED SUBSTANTIALLY UNIFORM DENSITY FLOOD BEAM OF ELECTRONS, AND A FLUORESCENT VIEWING SCREEN POSITIONED TO RECEIVE ELECTRONS OF THE FLOOD BEAM WHICH PASS THROUGH SAID STRUCTURE, MEANS FOR MODULATING THE WRITING ELECTRON BEAM OF SAID TUBE BY MEANS OF INPUT SIGNALS, AND MEANS FOR APPLYING BETWEEN THE BACKING GRID OR MESH AND THE CATHODE OF SAID FLOOD GUN SUCCESSIVELY DIFFERENT RECURRENT POTENTIAL VALUES SO CHOSEN IN RELATION TO THE SECONDARY EMISSION RATIO-VOLTAGE CHARACTERISTIC CURVE OF SAID INSULATOR THAT, IN THE ABSENCE OF STORED 